Industry has long attempted to make use of the bi-products of its basic processes. More recently, greater concern has been shown as to the effect of by-products of many processes being introduced into the environment. The foregoing has been especially true as applied to power generation operations, the most significant of such being coal-fired power stations. The waste gases produced by power stations, especially coal-fired power stations, are washed with the primary objective being the removal of sulphur. Generally, sulphur is removed by absorption using limestone (CaCO.sub.3) or lime in oxide (CaO) or hydroxide Ca(OH).sub.2 form. The residues from these processes contain separated sulphur dioxide (SO.sub.2) primarily in the form of calcium sulphite (CaSO.sub.3.xH.sub.2 O). The sulphite accumulates in suspensions or slurries and may also contain dust if the lime or limestone which simultaneously serves to remove dust and SO.sub.2 is added to the wash-separator. The calcium sulphite can however also exist relatively dust-free, if electrostatic filters are used before the washing. The residues exist in relatively large amounts and are difficult to handle, owing to their consistency and to their chemical properties. In particular there are considerable difficulties associated with disposal and use of the residue. The residues are worked up either to yield materials termed calcium alpha-sulphate hemihydrate or calcium beta-sulphate hemihydrate. In detail, an aqueous solution obtained in a wash of the combustion gases, which has absorbed the sulphur dioxide contained in gases of combustion, is the starting material. By using lime in the wash, CaSO.sub.4 x2H.sub.2 O (calcium sulphate dihydrate) is obtained. The residues which are thus obtained from the lime wash are oxidized with air in a vessel. The treated residues are transferred to a thickener. Lime sludge CaSO.sub.4 x2H.sub.2 O (calcium sulphate dihydrate) is obtained from the thickener. If the thus-obtained crude gypsum is passed to a drying apparatus, calcium beta-sulphate hemihydrate is obtained by calcination.
In order to obtain calcium alpha-sulphate hemihydrate, water is added to the crude gypsum in a mixer. The thus-obtained crude gypsum-aqueous sludge is pumped continuously into an autoclave and recrystallised into calcium alpha-sulphate hemihydrate gypsum under controlled conditions (temperature less than 150.degree. C.). Additives for control of the pH value and for alteration of the crystallisation crop of the alpha-sulphate hemihydrate can be introduced into the autoclave and facilitate the production of alpha-sulphate hemihydrate, with varying properties, as the end product. This alpha-sulphate hemihydrate is continuously drawn off from the aqueous phase in a centrifuge. On drying, e.g. in a circulating drier, powdered calcium alpha-sulphate hemihydrate (CaSO.sub.4.1/2H.sub.2 O) is obtained.
Both products, calcium alpha-sulphate hemihydrate and calcium beta-sulphate hemihydrate, are suitable for use as underground tunnel reinforcement material on account of their setting capability and strength. Owing to its lower strength, the beta-sulphate hemihydrate is of relatively restricted utility, particularly in back filling. Relatively high hardness and brittleness of alpha-sulphate hemihydrate, in its pure form, must be taken into account while using it as a construction material. These properties allow only limited utility for alpha-sulphate hemihydrate, but it has been found that alpha-sulphate hemihydrate with added flue dust from coal-burning plants as an impurity imparts the desired flexibility and thereby allows the wide use of alpha-sulphate hemihydrate as a building material for underground tunnel reinforcement. (In addition, these materials can be used within the construction industry and in many other areas where a reinforcing material is desired, especially where case-hardening underground materials are needed, such as building foundations, dams, etc.)
The flue dust employed to modify the properties of the sulphate hemihydrate can vary considerably in composition dependent upon the material being burned, the temperature employed, etc. However, the primary components of dust obtained from mineral coal are:
SiO.sub.2 45-55% by weight CaO 2-7% PA0 Al.sub.2 O.sub.3 23-35% by weight K.sub.2 O 3-5% PA0 Fe.sub.2 O.sub.3 3-10% by weight MgO 1-3%
Generally, the range of the ratio of sulphate hemihydrate to flue dust is 40-100 percent by weight to 60-0 percent by weight, respectively. A narrower range of 8-12% by weight of flue dust is desirable in certain applications.
The use of construction materials requires numerous transfers, and has associated handling problems. The large quantities of materials used and low costs required per unit quantity purchased have caused great attention to be directed to improving methods for transferring these materials. In the case of semi-solid flowable construction materials, a very desirable method for transporting these materials utilizes the concept of pneumatic stowing or conveying. This approach involves either the use of air under pressure to push materials from one point to another or vacuum systems to draw material from one point to another. Various prior art arrangements and systems for pneumatic stowing of construction materials are known to those who are skilled in the art; such prior art arrangements are not crucial to the present invention and are not dealt with herein. Thus, for the underground use of calcium alpha-sulphate hemihydrate and/or calcium beta-sulphate hemihydrate as a semi-solid construction material, pure or mixed with flue dust, and also for the use of this building material above ground, pneumatic stowing may be resorted to with advantages. The pulverised construction material comprising hemihydrates is generally conveyed pneumatically from a silo to the casing station and diluted as desired by the addition of a large amount of water when pneumatic stowing is used.
It has been found that in the pneumatic stowing of pure calcium alpha-sulphate hemihydrate or calcium beta-sulphate hemihydrate, blockages are formed if the pneumatic stowing is conducted over a long stretch. In addition, this problem occurs widely if the calcium alpha-sulphate hemihydrate and/or calcium beta-sulphate hemihydrate is mixed with flue dust, i.e. when a mixture of calcium alpha-sulphate hemihydrate and/or calcium beta-sulphate hemihydrate with flue dust is used as the constructional material which is pneumatically stowed. Lastly, the mixture with flue dust in addition to having occasional operation interruptions which are caused by non-uniform batch mixtures, one occasionally observes at evenly recurring intervals, flue dust escaping to an increasing degree. According to the invention, a considerable proportion of flue dust is led back into the sulphate hemihydrate powder stream, which appears in an apparently unobvious form, perceptible solely from the varying volumetric concentrations.
It is highly desirable to provide a manner of overcoming the blockages of pneumatically stowed construction material, especially when the construction material contains flue dust.
This invention has as its object the elimination of the blockages of pure calcium alpha-sulphate hemihydrate and/or calcium beta-sulphate hemihydrate as well as the interruptions in operation during pneumatic stowing of such construction materials with additions of flue dust. According to the invention, this is achieved by addition of large-grained natural form of anhydrous calcium sulphate (natural anhydrite) and/or limestone. Calcium alpha-sulphate hemihydrate or calcium beta-sulphate hemihydrate is thereby no longer in pure form but mixed with predetermined quantities of natural anhydrite or limestone or provided.
According to the invention, advantage is taken of the fact that added natural anhydrite or limestone does not react with the water added in the pneumatic stowing. The added water is bound to the calcium alpha-sulphate hemihydrate and/or calcium beta-sulphate hemihydrate, before natural anhydrite and limestone can react. This also applies in the case when the natural anhydrite is mixed with sulphates as reaction accelerators, such as FeSO.sub.4. The large particles of the additive according to the invention cause a composition behavior which allows considerably greater transport stretches in contrast to the pneumatic stowing of pure calcium alpha-sulphate hemihydrate and/or calcium beta-sulphate hemihydrate; the additive promotes an even distribution of the flue dust in the calcium alpha-sulphate hemihydrate and/or calcium beta-sulphate hemihydrate, thereby obviating blockages in pneumatic stowing.
For introduction into underground tunnel reinforcement or for similar uses, the addition of natural anhydrite and/or limestone is preferably in the range of 8-22% of the total introduced mixture, on a weight basis. Greater added amounts reduce the strength of, e.g. roadside packs, to an undesirable degree. Amounts of additive lying below, about 8% by weight, are not effective enough in increasing the transport mobility or flowability of the construction mixture.
A preferred percentage range for the natural anhydrite and/or limestone is from about 10 percent to about 12 percent on a weight basis.
In a preferred composition according to the invention, the tunnel construction reinforcement material would contain approximately 92 to 78 parts by weight of calcium alpha-sulphate hemihydrate and/or calcium beta-sulphate hemihydrate to about 8 to 22 parts by weight of natural anhydrite and/or limestone.